1. Field of the Invention
The invention relates to an image processing apparatus to perform face-detection processing, an image processing method and a control program used for the same purpose.
2. Description of the Related Art
An image processing method to automatically detect a particular pattern of a subject from an image is useful for its application, for example, of determining human faces. Possible fields of application of the method are so broad as to include multimedia communication conferences, man-machine interfaces, security systems, monitoring systems to track human faces, and image compression techniques.
Methods as follows are used as such a technique to detect faces from images. For example, a method uses some of the distinctive features of human faces and the geometrical relations peculiar to the positions of such distinctive features. An other method, such as the template matching method, uses the symmetric features of human faces and/or the characteristic colors of human faces. There is even a human-face detection method that uses the neural network.
When the face of a subject person is detected from an image, the person does not always show his/her full face with his/her head upright. Images are taken, using a camera, from various directions and in various magnifications. So face-detection techniques have to handle faces taken from various angles and in various sizes in images. Accordingly, detection processings have to be repeated with small adjustments made in the angle of a detector, the size of detection area, and the position for the detection.
There are an enormous number of possible combinations of the detector angle and the detection-area size. If the detection processings have to cover all of these enormous combinations, it takes a long time to finish the processings. Various methods have been proposed to address this problem. Some of the proposed methods reduce the detection processes by limiting the area to be searched. For example, a method limits the searching area by limiting the angle and the size of the face on the basis of the result of the first detection (Japanese Patent Laid-Open No. 2007-233517). An other method limits the searching area by predicting the angle and the size of the face on the basis of the detection result of the first image in successively captured images (Japanese Patent Laid-Open No. 2009-026299).
Note that the method of Japanese Patent Laid-Open No. 2007-233517 limits the searching area on the basis of the result of the first detection. So, if the result of the first detection is incorrect, the area of the face cannot be detected correctly in the following detections. The method of Japanese Patent Laid-Open No. 2009-026299 is unable to limit the searching area if the target images are not taken successively. So, in this case, the processing time cannot be shortened. Under the circumstances, face-detection processing by an appropriate method which shortens the time for detection processing and which reduces the occurrence of incorrect detection is desirable.